Paterson's version does something similar - the tube rides on a support drive roller at one end.
If starting from scratch, this is what I'd do...

Have a tank - above it a drive unit; floating in the water, a cylinder. The drive unit folds down and straddles the tank and the cylinder floats up under it with positive buoyancy, thus doing away with support bearings, seals, leaks, etc.
So simple.